Heretofore, there has been proposed and applied a power-assisted bicycle arranged to assist a pedaling force by detecting a pedaling torque acting on a rotational mechanism in accordance with the pedaling force and applying an electric torque on the basis of a variation in the pedaling torque detected. Such a power-assisted bicycle is provided with a torque detection system for detecting the pedaling torque, which includes a torsion bar mounted in a crank shaft or on a rear wheel shaft, which causes a torsion deformation in accordance with the pedaling force. Further, this torque detection system may utilize a technology that involves detecting a variation in resistance of a potentiometer in association with a direct linear movement, as an input torque, into which a torsion deformation of a torsion bar is converted by means of a cam or any other suitable element connected to the torsion bar or that involves directly detecting an angle of torsion rotation of the torsion bar.
These technologies, however, present a problem in that, due to the installment of the torsion bar on the crank shaft or the like, a large space is required for installing the torque detection system and the weight of the torque detection system is increased, making it necessary to considerably modify the body frame of a conventional bicycle. Another problem is that the cam or any other suitable element has to be disposed in order to convert the torsion deformation of the torsion bar into a direct linear movement, or a system for directly detecting an angle of rotation by the torsion from each of the both ends of the torsion bar is required to be added. These matters may also create a problem in that the mechanism is rendered more complicated.
Therefore, in order to render the weight of the torque detection system lighter and the structure thereof simpler, there has been proposed a technology that can detect a crank-axial displacement, as a pedaling torque, by locating a conversion system for converting a rotational phase difference into an amount of the crank-axial displacement. The rotational phase difference is caused to arise between a driving part and a driven part, and the driving part is arranged so that the pedaled force acts directly thereon from the crank shaft side and the driven part being connected to the driven part and arranged so as to transmit the pedaled torque to a wheel rotation mechanism system.
For instance, Japanese Patent Laid-open Publication No. H8-230,756 discloses the technology of detecting a variation in distance of a disc member with respect to a frame as a physical amount corresponding to the pedaled torque, the variation in distance being caused by elongation of a coil spring in the crank-axial direction in accordance with the rotational phase difference between both ends of the coil spring by means of the pedaled torque. In this system, the disc member (e.g., a sprocket, etc.) for transmitting an input to the pedal received from the crank-shaft side to another mechanism system is supported on the frame so as to be rotatable and movable in the axial direction in a state in which the disc member is disposed coaxially with the crank shaft. Moreover, the coil spring is disposed in such a manner that one end portion thereof is fixed to the crank shaft while the other end portion thereof is fixed to the disc member in such a manner that the crank shaft is inserted inwardly.
Further, Japanese Patent Laid-open Publication No. H10-76,987 discloses a torque detection mechanism system composed of a drive disk connected directly to a drive shaft, and a follower disc plate connected directly to a sprocket. Two sheets of U-shaped spring plates are interposed between the drive disk and the follower disc plate, and a cam part is disposed at a connection of an outer periphery between the drive disk and the follower disc plate which are engaged slidably with each other so as to convert a rotational phase difference between the drive disk and the follower disc plate into an axial displacement. A slide cap is disposed so as to envelop the drive disk and slide in the axial direction together with the drive disk, a potentiometer is provided for detecting the displacement in the axial direction in a state in which a contact thereof is engaged with the slide cap, and a coil spring is provided for biasing the slide cap in the axial direction toward the driven disk side.
The torque detection mechanism system as disclosed in Japanese Patent Laid-open Publication No. H8-230,756 can achieve the purpose of simplifying the mechanism. However, it does not adequately solve the problem of saving space for the torque detection mechanism system due to the elongation of the crank shaft as a whole in the axial direction, because the coil spring is supported by the frame coaxially with the crank shaft, the coil spring being arranged so as to be elongated or contracted in the axial direction due to expansion or contraction of its diameter in accordance with the magnitude of the pedaling torque. Moreover, the addition of the heavy and large-sized coil spring presents a problem with the lightweight structure of the torque detection mechanism system because the coil spring is high in rigidity and in modulus of elasticity so as to withstand the pedaling force. Furthermore, this bicycle has a problem with a feeling of resistance when pedaling since it remains when pedaling.
Further, for the torque detection mechanism system as disclosed in Japanese Patent Laid-open Publication No. H10-76,987, the cam part is provided and a drive disk and a follower disc plate are added, which are arranged so as to slide in the axial direction in accordance with a rotational phase difference. This arrangement, however, cannot save a sufficient amount of space or be of a structure lightweight enough for the torque detection mechanism system, similar to the prior technology described immediately above.